The present invention relates to apparatus for heat-softening and attenuating glass material into fibers, and more particularly to an electrical resistance furnace known in the art as a bushing. A bushing is used to melt, condition and fiberize the glass material.
Prior art bushings, whether utilized in a direct-melt or marble-melt configuration, are almost always constructed of platinum-rhodium alloys. Such alloys are used because they are highly corrosion resistant and can accommodate high operating temperatures in excess of 2450.degree. Fahrenheit. In prior art marble-melt bushings, it has generally been the practice to provide substantially greater temperatures in the melting and conditioning of glass material than is actually necessary for fiberization. Specifically, as set forth in U.S. Pat. Nos. 3,013,095 and 3,048,640, molten glass must be heated at a temperature upwardly of 2300.degree. F. This same teaching is also set forth in U.S. Pat. No. 3,615,314 wherein it is stated that a bushing must be maintained generally at a temperature of about 2400.degree.-2500.degree. F. It must be emphasized that if the glass is to be maintained at such temperatures, upwardly of 2300.degree. F., the side walls of the bushing, as well as the top wall, will be at an even higher temperature.
However, it has been determined that actual fiberization temperature of most common glasses is not in excess of 2250.degree. F. Prior art bushing constructions have required that the side wall temperatures be several hundred degrees higher than the base plate temperature in order to maintain the base plate temperature at the fiberization temperature, which may be in the order of 2000.degree.-2250.degree. F. Higher side wall temperatures are required because the base plate will radiate heat off into the atmosphere. However, the overall effect is to provide a bushing which is of "high heat" mode, i.e. a bushing in which temperatures above 2250.degree. F. occur.
Prior art bushing constructions, in the marble-melt bushing mode, also generally utilize a construction in which the side walls are joined at a seam at their upper ends to a basket. The basket serves to retain glass material during the melting phase. However, the seam does not generally contact glass material and therefore provides a juncture of extremely high temperature. This can lead to bushing failures especially in the case where it is desired to construct a bushing of a nonplatinum-rhodium alloy.
Accordingly, it is a general object of the present invention to provide an electrical resistance bushing for use in an apparatus for heat-softening and attenuating glass material into fibers, and more particularly, it is an object to provide a low temperature bushing. This is achieved by a construction which ensures that the base plate and side walls are maintained in approximately the same temperature range, i.e. in the range of approximately 2000.degree.-2250.degree. F. Maintaining this temperature range is accomplished by a novel dimensioning of the relative thicknesses of the base plate and side walls. In the case of a marble-melt bushing, dimensioning of the top wall relative to the base plate is also an important criteria.
Specifically, a low temperature bushing of the present invention is accomplished in the direct-melt mode by dimensioning the base plate with an average thickness at least four times greater than the average thickness of each side wall. With respect to a marble-melt bushing having a top wall, the present invention contemplates that the base plate will be dimensioned with an average thickness of at least eight times the average thickness of an upper section of each side wall over at least about fifty percent of the vertical dimension, measured along the length of the bushing. The base plate is also dimensioned to be at least eight times as thick as the average thickness of the top wall.
With the above set forth dimensions as a criteria for direct-melt and marble-melt bushings, it has been found that the base plate and side walls may be maintained at approximately the fiberization temperature for most common glasses. This results in a low temperature bushing enabling the use of materials alternative to platinum-rhodium alloys. Exemplary materials may include nickel-chromium alloys, stainless steels, nickel-tungsten alloys, etc.
Another object of the present invention is to provide a direct-melt or marble-melt bushing with a relatively thick base plate, as described above, with the additional provision of auxiliary end plates mounted adjacent each end wall of the bushing. Specifically, each auxiliary end plate will be spaced from its adjacent end wall and secured to an adjacent terminal and to the base plate. Because each auxiliary end plate is spaced from its adjacent end wall, electric current will be channeled from the terminal directly to the base plate without straying through the lower corners of the side walls thereby eliminating deleterious effects on the temperature pattern of the bushing. This also results in a substantially uniform base plate temperature.
Still another object of the present invention is to provide a marble-melt bushing, dimensioned as described above, with a plurality of material-directing funnels which are mounted on the top wall of the bushing. Surrounding a peripheral portion of the funnel, adjacent its connection to the top wall, is a cooling means provided for maintaining a temperature which will prevent glass material from adhering or accumulating in a region adjacent its entry through an opening into the interior of the bushing. It is contemplated that a collar will be positioned between the end of the funnel and the opening in the top wall which is cooled by the cooling means.
A still further object of the present invention is to provide a marble-melt bushing, dimensioned as described above, with opposed side walls which are inclined and extend upwardly and outwardly from the base plate. Each of the side walls is joined, at its upper end, to an upper side wall section which is inclined inwardly toward the longitudinal axis of the bushing. This construction results in the provision of a relatively narrow top wall. Furthermore, the outwardly inclined side walls provide a structure which serves to "bridge" molten material as it is being melted from a solid condition to a more viscous condition.
These and additional objects and advantages of the present invention will be more readily understood from a consideration of the drawings and the detailed description of the preferred embodiment.